Laser systems in the art invariably utilize reflectors to control the direction and some of the polarization properties of a usually monochromatic radiation emitted from a laser source. Reflectors, therefore, are known in the art. One of such prior art reflectors is shown in FIG. 1A. That reflector has thin film optical coatings on it for improving the reflectance and/or polarization of an incident light beam. In addition, that reflector is designed for use with a beam of specific wavelength. More particularly, that reflector comprises a substrate onto which a highly reflective metallic layer such as silver or aluminum is deposited. A thin film optical coating is generally deposited atop the metallic layer to protect the reflector from environmental hazards such as humidity, scratches, etc. The use of this thin transparent dielectric film contributes to the nomennclature "protected silver reflector."
The protected silver reflector has an inherent characteristic, i.e., the relative or differential phase between the "p" and "s" linear polarizations of an incident beam is shifted four to six degrees during each reflection. Such phase shift or retardation is generally acceptable when generally three or less reflections are required in such a laser system. Where several reflectors are required in a system, however, the polarization of the beam is so significantly altered that the resultant beam is undesirable. An example of such a multi-reflection laser delivery system is shown in FIG. 3, in which eight reflectors are employed. If the work to be performed by the laser system of FIG. 3 is to cut or machine a thick piece of metal into a configuration, and especially when either the workpiece or the laser delivery arm needs to be maneuvered, the quality of the cut will be different at different locations of the cut since the cut is dependent on the polarization of the laser beam. This is a well recognized phenomenon of laser cutting.
In order to enhance the reflectance or percent of light reflected off a protected silver reflector such as the one shown in FIG. 1A, another prior art reflector such as the one illustrated in FIG. 2A is used. The reflector in FIG. 2A also comprises a substrate onto which a highly reflective metal is deposited. Instead of just one dielectric layer, several alternating layers of dielectric thin films are deposited; the material of one type of the dielectric thin films is a high optical refractive index material such as Ge or TiO.sub.2, and the other type being a low optical refractive index material such as SiO.sub.2, ZnS or ThF.sub.4. Each of these thin film layers has an optical thickness of one quarter wave of the wavelength of the laser light, contributing to the nomenclature "quarter wave stack." This type of reflector is also generally referred to as an "enhanced silver reflector," and an example of which is disclosed in Fischer et al., U.S. Pat. No. 4,379,622. Enhanced silver reflectors are designed to give a reflectance higher than that of the protected silver design by using quarter waves of alternating low and high refractive index materials. In addition, enhanced silver reflectors are also capable of preserving incident polarization if the angle of incidence of the light beam is less than approximately 40 degrees. This feature, however, was neither recognized nor sought by those skilled in the art. Moreover, if the thicknesses of the layers are not correctly tuned, i.e., tuned to the center wavelength of the beam, then an arbitrary differential phase shift will occur. This infirmity is similar to that of the protected silver reflectors.
Reflectors in the present art, when used in systems such as the one illustrated in FIG. 3, are positioned such that the angle of incidence of the laser beam is at 45 degrees. Although constraining the angle of incidence of all the reflectors to 45 degrees eliminates mechanical problems of beam alignment, it increases the number of reflectors required and the power loss in the laser delivery system. Such a constraint is, therefore, undesirable.